Conventional hot forging of iron, nickel, titanium and aluminum alloys exploits the characteristic common to all metallic materials that resistance to deformation decreases rapidly with increasing temperature and consequently, for a given forging installation, larger diameter blanks can be formed if the workpiece temperature is increased. This generalization, however, is limited by a number of natural factors: (1) the temperature at which the alloy begins to melt; (2) possible phase transformations which can lead to deterioration in the properties of the finished product; (3) the poor workability of many high temperature materials such as some super alloys; (4) changes in temperature during the deformation process as a result of heat transfer from the hot workpiece to a relatively cold forging die set; and (5) the stresses to which the tooling may be safely subjected.
U.S. Pat. No. 3,519,503 describes a method of forging high strength, difficult to forge alloys by placing them in a temporary condition of low strength and high ductility, In the U.S. Pat. No. 3,519,503 method the alloy being forged must undergo a series of heat treatments to return it to a high strength condition after forging. The U.S. Pat. No. 3,519,503 method requires that the alloy have a fine grain structure of less than 35 microns during forging, but to achieve the desired high strength and high temperature characteristics in the forged article the grain structure must be altered by a heat treatment which will cause the grain size of the forged article matrix to increase significantly. The method disclosed and claimed in U.S. Pat. No. 3,519,503 is directed to processing high strength, high temperature, difficult to forge titanium and nickel super alloys. Similar processing techniques are also discussed in "Isothermal Forming a Low Cost Method of Precision Forging" Engineering (July 1979); "Superplastic Behavior during Compression of As-Cast Al-CU Eutectic Alloy", Metals Technology, p. 355, Vol. 9 (September 1982); "Precision Aluminum Forgings Trim Weight, Maintain Strength", Machine Design p.63, (Nov. 10, 1983); and "Precision Forging of a High Strength Superplastic Zinc-Aluminum Alloy", Metallurgical Transactions A, p. 1259, Vol. 9A (September 1978).
Aluminum forgings have been used in aircraft structures because aluminum provides light weight, while the grain flow of the metal in the forging imparts high strength, ductility, and resistance to impact and fatigue because it follows the contour of the shape being forged. Regardless of their advantages, conventional aluminum forgings have in recent years been losing out to advanced composites and titanium forgings in airframe specifications. Thus, it can be seen that there remains a need for more sophisticated forging techniques which will allow aluminum to regain its past position in airframe specifications, particularly in the area of near net sized parts.